If (3) is violated, (4) involves a division-by-zero and the ACE is no longer nonparametrically estimable (a similar problem occurs for nonparametric g-computation). Even if non-zero, \(\widehat{\Pr }_n(A=1 | \mathbf {Z}_i)\)  values close to zero can lead to instability.[1]}, [2]}, [3]}, [4]}
